The present invention relates to a method of driving an ink-jet head of a multi-drop system which unifies a plurality of ink-drops sequentially emitted from an orifice to form a one-dot liquid drop.
There has been a method of expressing gradations by using an ink-jet head which performs printing in such a manner in which a plurality of ink chambers are provided, drive pulse voltages are applied to piezoelectric members provided in correspondence with the ink chambers so as to cause the piezoelectric members to be transformed, and the ink chambers are selectively deformed by the transitions of the piezoelectric members so as to emit ink from the ink chambers. A method known as the kind of method described above is, for example, a method of expressing gradations in which sizes of ink drops hit on a recording medium are changed by controlling volumes of ink drops to be emitted by PWM (pulse-width-modulation) control, as disclosed in U.S. Pat. No. 5,461,403, or a method of expressing gradations in which a plurality of ink drops are emitted sequentially from one same orifice and the number of ink drops hitting on the same portion on a recording medium is controlled.
The former method has a problem that the emission volumes of ink drops are not constant unless the following ink drop is emitted under a condition that the meniscus of the orifice is recovered and stabled to some extent after emission of a previous ink drop from the orifice. Therefore, the driving frequency must be lowered, so that the printing speed is difficult to be increased. In contrast, the latter method of a multi-drop system is advantageous in that it is possible to improve the printing speed by increasing the driving frequency and that small liquid drops can be emitted without reducing the emission speed. However, since a line head performs printing while moving a recording medium in the sub-scanning direction, for example, there is a problem that the recording medium itself is moved and the printed dots are elongated in the moving direction of the recording medium while sequentially emitting seven liquid drops in the case of performing one-dot printing by emission of seven liquid drops.
A method which solves the above problems is, for example, a method in which the speed of ink drops to be emitted later is gradually increased from the speed of the ink drop to be emitted first, such that the ink drops emitted later catch up with and are merged with the ink drops emitted earlier to obtain one liquid drop therefrom when the ink drops hit a recording medium. This is realized by applying successive drive pulse voltages to a piezoelectric member such that the amplitude of a pressure wave in an ink chamber is gradually increased when emitting ink.
However, in this case, the vibration amplitude of an ink chamber increases since the emission speed of ink drops to be emitted later is increased. In some cases, the vibration may influence an adjacent ink chamber so that ink may be erroneously emitted from the adjacent ink chamber. In order to avoid this problem, the adjacent ink chamber may be set as a dummy ink chamber which is not allowed to perform ink emission. Even in this case, if the ink chambers situated in both sides of a dummy ink chamber inserted therebetween perform simultaneously ink emission operation, vibrations of ink chambers are transmitted to a common ink chamber which supplies ink in common to the ink chambers. In particular, if the entire line head is brought into a condition that the ink chambers situated in both sides of the dummy ink chamber perform simultaneously ink emission operation, the pressure of the common ink chamber is greatly changed due to transmission of vibrations, and as a result, ink emission conditions of the ink chambers are respectively changed thereby causing variation of printing results.